PERIODIC VARIATIONS IN ULTRAVIOLET SPECTRAL-LINES OF THE B0.5-IB STARHD-64760 - EVIDENCE FOR COROTATING WIND STREAMS ROOTED IN SURFACE VARIATIONS

We discuss recently observed periodic modulations in the UV wind lines of the B-type supergiant HD 64760, with a focus on the peculiar, upwa rdly bowed shape seen in isoflux contours of the absorption variations plotted against velocity and time. We show that this qualitative impr ession of bowed contours is quantitatively confirmed by a peak in the phase for the associated periodic variation at very nearly the same li ne position as the apparent bow minimum. The bowed shape is significan t because it indicates that wind variations evolve both blueward and r edward, i.e., toward both larger and smaller line-of-sight velocities. We show here, however, that these characteristics arise naturally fro m absorption by strictly accelerating corotating wind streams seen in projection against the stellar disk. The quite good agreement obtained with the observed profile variations provides strong evidence for cor otating stream modulations in this wind.